rpcapd: make a shift count unsigned.

rpcapd: make a shift count unsigned.

That should prevent compilers and, more important, static analyzers (I'm
looking at *you*, Coverity!) from saying "this converts the unsigned
char to an int, which somehow taints it, so I get to use my favorite
meaningless word "tainted" in a warning".

(What does "tainted" mean to Coverity, anyway?  ZOMG UR READING NUMBERZ
FROM AN EXTERNAL SOURCE!!!!!!!!1111ONE!!!!!!!!!?)

(cherry picked from commit b1f9b176115eae9235ac4fd48aa0da9c61f46e67)